The present invention relates to articulated seabed mattresses for the protection and stabilisation of seabed installations such as pipelines.
The use of articulated mattresses in offshore coastal and marine engineering is well known for stabilisation, protection and scour prevention of pipelines, flowline umbilicals, seabed templates, steel and concrete platforms and the like. These mattresses are particularly useful in areas of high bottom current where hydro-dynamic forces are considerable. Thus, for example, a seabed pipeline can be covered with such a mattress so that the pipeline is stabilised by the weight of the blanket thereon and also the adjacent seabed is protected against erosion. Examples of articulated mattresses and their use are given for example in European patent specification 152232A. Generally, these mattresses comprise concrete or similar elements joined together to allow relative articulation.
Subsea stabilisation mattresses can be relatively massive, eg. a mat of 5 m.times.2 m would weigh (in air) over 2.5 tons. However, even so, when they are laid over a seabed pipeline, for example, they can still be prone to movement during a storm or tidal surge. While, in theory, greater stability could be obtained by increasing the mat weight, we have found another way of dealing with this problem.
Further, in addition to stability in extreme storm conditions, a stabilisation mattress should desirably also be capable of withstanding the impact thereon of anchors or trawlboards travelling laterally, or dropped objects travelling largely vertically.
We have now devised a new design of sub-sea mattress which can provide substantial advantages in use over prior known mattresses.
In accordance with one aspect of the present invention, there is provided an articulated mattress for laying on a seabed or the like, which comprises a plurality of concrete or like members articulated together, characterised in that the mattress has a relatively thick region from which its thickness tapers to at least one side edge.
In a simple embodiment, a mattress of the invention is generally wedge-shaped. That is to say, when laid on a horizontal surface, its transverse sectional shape is roughly that of a right-angle triangle. More usually, however, mattresses of the invention will have a shape generally similar to that of a ridge tent, i.e. with a central elongate thick region tapering down on each side to a side edge of the mattress. Another possibility is to have a generally pyramidal shape, i.e. a central thick region tapering down on all sides to the edge(s). In all cases, the upper sloping surface(s) may be generally flat, or generally curved, eg. concave or convex as desired. Whilst the commonest mattress shape in plan is generally rectangular, in fact the mattresses can be of any desired plan shape as required in practice.
By providing the mattress with its own taper from a thin edge to a much thicker region, improved stability can be achieved and also improved ability to withstand lateral impact by dragged anchors or trawlboards, for example, and by dropped objects descending vertically.
The mattresses of the invention taper in thickness from a relatively thick region to at least one side edge. Normally, the tapering extends through at least two of the constituent elements of the mattress.
Conventionally, when a sea-bed pipeline or other object is to be protected, a subsea mattress is laid thereover. In accordance with a highly preferred feature of the present invention, the underside of a mattress is provided with a recessed region in which the object to be protected is received. Such a recessed region is most advantageously provided in the thick region of the mattress. This gives excellent protection to the object and also allows certain further preferred features of the invention to be adopted (described hereinafter).
The mattresses of the invention are constructed from massive elements formed from concrete or other weighty material. The concrete can be adjusted to have a low specific gravity, 1.5 t/m.sub.3 for high conditions where seabed soils have little bearing strength, up to 4.7 t/m.sup.3 for high current applications where additional weight is required to cope with extreme seabed currents which can be as high as 10 m/s. The mattress elements are connected by rope, hinge, geotextile or other flexible connection mechanism so that the mattress folds during deployment and the joints between the elements allow sufficient flexibility to accommodate normal seabed discontinuities.
In the mattresses of the invention, the relatively thick region will normally be made of correspondingly thick concrete (or the like) elements with gradually thinner elements being used towards the edges to give the desired taper. the outermost elements will most preferably be shaped to give protection against scour.
In a much preferred arrangement of the invention, the thick region of the mattress (which will normally be constituted by one element, although a group of two or more can be used) has on its underside a recess to receive the object to be protected, eg. part of a pipeline. Thus, such a mattress is placed over the pipeline which is received in the recessed region. This region of the pipeline is preferably encapsulated and so protected against environmental and damage loads. The pipeline need not necessarily touch the mattress at all. Thus, the thick recessed region of the mattress can straddle the pipeline without contacting it.
For certain applications where thermal expansion may be substantial and considerable movement within the recess is anticipated, or the recess is required to bear directly onto the pipeline, and when the pipeline has a special coating such as an insulating coating, the recess is preferably coated with a suitable anti-abrasion coating which could for example be a high build paint, or sheet of material such as polypropylene or polyester suitably attached to the inner surface of the recess.
At the bottom of the mattress, at the edge of any recess, there is preferably a cusp to ensure that the pipeline is retained within the recess regardless of the extent of any lateral movement. This cusp may be extended for certain applications so that it penetrates the seabed locally to enhance its ability to retain the pipeline within the recess.
The individual mattress elements can, for example, incorporate a contoured underside to increase the frictional resistance of the seabed and thereby provide additional lateral restraint.
The individual mattress elements are preferably slab-sided to ensure that adjacent matteress elements do not fold or buckle upward when a lateral force is applied. The upper surfaces of the individual elements can be contoured to reduce the hydro-dynamic lift on the elements when they lie in a lateral current flow. This flow would tend to destabilise and move the mattress in extreme water particle velocity conditions. The overall profile of the mattresses of the invention, i.e. progressive tapering from the thick region (preferably central) to the edge(s), enhances the stability of the mattress by providing a positive pressure build up on the upstream face which enhances the frictional effects which resist lateral movements and counteract the lift effects produced on the downstream face.
In recent times, it has become important to be able to pass hot materials through seabed pipelines without large heat losses. In order to achieve this, it has been necessary to use insulated pipes (which are expensive) and to locate the pipelines in trenches dug in the seabed. This is a very expensive installation and, moreover, suffers certain other disadvantages. In particular, pipelines which carry hot materials suffer substantial thermal expansion and contraction, and this has resulted in buckling of the pipelines in trenches and in damage to the pipelines themselves. We have now found, in accordance with a preferred feature of the present invention, that the mattresses of the present invention with a tunnel in their thick region, are excellent for protecting high-temperature pipelines. Use of such mattresses can obviate completely the necessity for trenches. The tunnels allow for lateral movement of the pipelines, to take up thermal dimensional changes. Also, the tunnel itself provides some small amount of thermal insulation.
In accordance with a further preferred feature of the invention, the tunnel can be filled with a heat insulant to further protect the pipeline against thermal losses. Thus, the insulation is provided to the pipeline on the seabed after the mattress has been laid on the pipeline. When a mattress of the invention is laid on a pipeline with the latter in the mattress tunnel, the space between the pipe and the mattress can be filled, or partly filled, with an insulating material. A preferred insulating material is a cementitious grout which can be flowed into the space between the pipe and the mattress, and allowed to set.
In many (but not all) cases, it is preferred to provide a flexible bag or other container for the insulation material, between the pipe and the mattress, for example to receive and contain a fluid grout in position while it cures. The container will normally be positioned in the tunnel to lie between the mattress and the pipe during installation of the mattress, and may thereafter be filled with insulant. Alternatively, and usually far less preferably, the container can sometimes be positioned after installation of the mattress.
Conveniently, the container will include at least one inlet accessible from outside the overlying mattress, for introduction of insulant into the container. A preferred form of container is a flexible bag, but other types of container can be used.
The principal function of any bags or other containers is to hold the grout or other insulant in position, especially during any curing thereof. The container is flexible to allow the insulant to conform to the shape of the pipe and so form a coating thereover. Another function of the containers can be to assist in relieving thermal stresses. Thus, as the temperature of a pipeline varies, so it expands and contracts. We have found, in accordance with a preferred feature of the invention, that by using a container of woven, rubber or another compressible material, the material itself (sandwiched between the insulation and pipeline) can absorb or reduce the effect of thermal stresses on the insulation coating.